The present invention relates to an image forming apparatus and an image forming method which repeat image create/transfer processing for a plurality of toner colors which are different from each other so that toner images in the respective toner colors are laid one atop the other on a transfer medium, such as a transfer drum and a transfer belt, and a color image is accordingly formed. As herein described, the xe2x80x9cimage create/transfer processingxe2x80x9d refers to a series of processes that after forming a toner image on a photosensitive member while rotating the photosensitive member and a transfer medium in a sub scanning direction, the toner image is transferred onto the transfer medium.
An image forming apparatus of this type is as that shown in FIG. 59, for example. This image forming apparatus allows to form toner images in a plurality of colors which are different from each other, e.g., four colors of yellow (Y), cyan (C), magenta (M) and black (K), on a photosensitive member 21 which is driven to rotate. The respective toner images are primarily transferred onto a transfer medium 41, such as a transfer belt and a transfer drum, which rotates in synchronization with the photosensitive member 21. The image forming apparatus comprises a drive source 81, such as a dynamotor and a pulse motor, in order to drive the photosensitive member 21 and the transfer medium 41 into rotation. Rotational drive force generated by the drive source 81 is applied to the photosensitive member 21 and the transfer medium 41 through a power transmission unit 9 which comprises power transmission members 91, such as a plurality of gears and a belt, and drives the photosensitive member 21 and the transfer medium 41 into rotation in mutual synchronization.
In this image forming apparatus, toner images in the respective colors are laid one atop the other on the transfer medium 41 through repeated image create/transfer processing for the plurality of colors, and a color image is formed on the transfer medium 41. Following this, the color image is secondarily transferred onto a sheet member S, such as a copy paper, a transfer paper, a paper and a transparent sheet for an overhead projector, which is fed from a cassette or manual-feed tray.
In order to obtain an excellent color image, it is necessary to lay toner images in the plurality of colors one atop the other while registering the toner images with each other. To this end, in the image forming apparatus described above, a sensor 40 for detecting a reference position of the transfer medium 41, for instance, is disposed in the vicinity of the transfer medium 41, and a signal which is outputted from the sensor 40 for every rotation of the transfer medium 41 is used as a reference signal for the image create/transfer processing. More specifically, after a toner image is created on the photosensitive member 21 at predetermined timing for every outputting of the reference signal, the toner images are primarily transferred onto the transfer medium 41 which rotates at a constant speed in synchronization with the photosensitive member 21. As a result, the toner images in the plurality of colors are laid over with each other accurately. Hence, the transfer medium 41 needs be driven to rotate at a constant speed in synchronization with the photosensitive member 21 until the primary transfer completes since outputting of the reference signal from the sensor 40.
However, abutting means 400, such as a secondary transfer roller for secondary transfer onto the transfer medium 41 and a cleaning part for cleaning of the transfer medium 41, sometimes temporarily comes into contact at proper timing, thereby changing loads upon the transfer medium 41, the power transmission members 91, etc. In other words, the contact could hamper the drive rotation of the transfer medium 41, elastically stretch the transfer medium 41, elastically bend the power transmission members 91 in a similar manner, or further, change a load upon a driving part (not shown) which drives the transfer medium 41 into rotation. The contact and separation could prevent the transfer medium 41 from rotating at a constant speed.
In an image forming apparatus of this type, in particular, in order to accurately transmit rotational drive force from the drive source 81 to the photosensitive member 21 and the transfer medium 41, gears formed by a resin material, such as polyacetal (POM), polycarbonate (PC), polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) and polyimide (PI), are often used, and therefore, the gears are elastically deformed as such loads described above change, which is one of main causes of a registration deviation. Further, where the transfer medium 41 is a transfer belt, stretching and shrinking of the transfer medium 41 caused by a change in the loads described above is one of main causes of a registration deviation. A registration deviation which is caused as the abutting means 400 contacts and moves away from the transfer medium 41 will be described in detail in the sections xe2x80x9cA-3. Analysis of Causes of Registration Deviationxe2x80x9d and xe2x80x9cB-3. Analysis of Causes of Registration Deviationxe2x80x9d later.
Causes of a registration deviation are not limited to these. A registration deviation may be caused by the following as well. That is, in an image forming apparatus of this type, the photosensitive member 21 and the transfer medium 41 are driven into rotation in mutual synchronization in a sub scanning direction. As the sensor 40 outputs a vertical synchronizing signal using this as a reference, a light beam scans over the photosensitive member 21 in a main scanning direction, which is approximately perpendicular to the sub scanning direction, based on an image signal which is supplied from an external apparatus such as a host computer, whereby an electrostatic latent image which corresponds to the image signal is formed on the photosensitive member 21.
Further, after the electrostatic latent image is developed by a developer with toner and a toner image is formed, the toner image is transferred onto the transfer medium 41 which is driven into rotation in synchronization with the photosensitive member 21 in the sub scanning direction. Such image create/transfer processing is executed for the respective toner colors (yellow, cyan, magenta and black), so that the respective toner images are laid one atop the other and a color image is created on the transfer medium 41.
However, in an image forming apparatus of this type, scan timing of the light beam is not synchronous to the vertical synchronizing signal in many cases, sometimes leading to a synchronization error between the vertical synchronizing signal and the scan timing. In this case, a transfer position on the transfer medium 41 becomes deviated by an amount corresponding to the synchronization error. Since synchronization errors become different among the respective toner colors, the toner images a become deviated from each other among the respective toner colors, that is, a registration deviation which degrades an image quality is developed.
The present invention has been made in view of the problem above, and accordingly, aims at providing an image forming apparatus and an image forming method with which it is possible to suppress a registration deviation on a transfer medium and form a high-quality image.
According to the present invention, based on a registration control amount which is needed to correct registration deviations which are caused as image create/transfer processing is repeated for a plurality of toner colors which are different from each other and toner images in the respective toner colors are laid one atop the other on a transfer medium, transfer start positions for toner images in at least one or more of the toner colors are corrected. This eliminates or suppresses relative registration deviations among the toner images on the transfer medium and improves an image quality.
One of causes of registration deviations is thought to be contact and separation of abutting means to and from the transfer medium. Noting this, according to the present invention, the abutting means is allowed to contact and move away from the transfer medium during repeated image create/transfer processing and transfer start positions for toner images are corrected using, as a registration control amount, a control amount which is needed to correct relative registration deviations among toner images on the transfer medium which are caused as the abutting means contacts and moves away from the transfer medium. This eliminates or suppresses registration deviations which are created as the abutting means contacts and moves away from the transfer medium and improves an image quality.
Further, according to the present invention, registration control amount establish processing is executed before forming a color image, in order to obtain a registration control amount which is needed to correct registration deviations which are created as the abutting means contacts and moves away from the transfer medium. The registration control amount establish processing may be to obtain a registration control amount with the abutting means contacting and moving away from the transfer medium which is rotating in a dedicated sequence which is different from a printing sequence which is used to form a color image, for instance. In this manner, it is possible to accurately identify a registration control amount which is essential to highly precise registration control.
Alternatively, the present invention further comprises abutting means which temporarily contacts a transfer medium during repeated image create/transfer processing in a sequence which corresponds to an operation state of the apparatus among a plurality of sequences which are different from each other; and memory means which stores in advance a plurality of registration control amounts which are necessary to correct relative registration deviations among toner images on the transfer medium which are caused as the abutting means contacts and moves away from the transfer medium. A registration control amount which corresponds to one sequence is read from the memory means and a transfer start position of a toner images is corrected for each toner color based on the registration control amount. Hence, it is not necessary to newly identify a registration control amount every time the sequence changes, and therefore, excellent controllability is achieved.
Alternatively, according to the present invention, registration control amount correction is executed after a color image is created based on a registration control amount at least once or more times, so that the registration control amount is corrected. While an operating environment, such as a temperature and a humidity level inside the apparatus, usually changes as color image generation proceeds thereby causing the registration control amount to deviate from an optimal value, since the registration control amount is corrected by means of execution of the registration control amount correction in the present invention, the registration control amount is optimized in accordance with an operating environment, etc. Hence, a color image is obtained more stably.
Other cause of the registration deviations is thought to be asynchronous control of the vertical synchronizing signal and the scan timing. Noting this, according to the present invention, driving means is controlled in accordance with a synchronization error period between the vertical synchronizing signal and the scan timing to thereby temporarily control acceleration/deceleration of at least the transfer medium and correct registration deviations which are attributed to the synchronization error period. This eliminates or suppresses registration deviations which are induced by the asynchronous control, and improves an image quality.
Further, according to the present invention, the image create/transfer processing is executed in response to the vertical synchronizing signal outputted from vertical synchronizing signal detecting means, and transfer start positions for toner images for the respective toner colors are corrected based on a first registration control amount, which is necessary to correct relative registration deviations among toner images on the transfer medium which are caused as the abutting means contacts and moves away from the transfer medium since the vertical synchronizing signal is outputted until the image create/transfer processing corresponding to this vertical synchronizing signal completes, and a second registration control amount, which is necessary to correct relative registration deviations among toner images on the transfer medium which are attributed to a synchronization error between the vertical synchronizing signal and the scan timing. The registration deviations of the two types described above are therefore suppressed at the same time, which in turn allows to obtain a color image having a higher quality.
Alternatively, for the purpose of eliminating registration deviations, the present invention further comprises driving means which drives a photosensitive member and a transfer medium into rotation in a sub scanning direction in synchronization with each other. During the correction, the photosensitive member and the transfer medium are accelerated/decelerated temporarily to a second driving speed from a first driving speed, and a position at which toner images are to be formed on the photosensitive member is shifted by a registration control amount in the sub scanning direction, whereby transfer start positions for toner images on the transfer medium are corrected in the sub scanning direction.
Alternatively, for the purpose of eliminating registration deviations, the present invention further comprises photosensitive member driving means which drives the photosensitive member into rotation in the sub scanning direction at the predetermined first driving speed and transfer medium driving means which drives the transfer medium into rotation in the sub scanning direction. During the correction, the transfer medium is accelerated/decelerated temporarily to the second driving speed from the first driving speed, whereby transfer start positions for toner images on the transfer medium are corrected in the sub scanning direction.
Alternatively, according to the present invention, the registration control amount establish processing is executed before forming a color image and a registration control amount, which is necessary to correct relative registration deviations among toner images on the transfer medium which are caused as the abutting means contacts and moves away from the transfer medium, is obtained from data which are acquired during the registration control amount establish processing, while when suspension of the registration control amount establish processing is removed, a registration control amount is obtained from data which are stored in a memory part without executing the registration control amount establish processing once again to thereby correct transfer start positions for toner images for the respective toner colors in accordance with the registration control amount. This realizes the following functions and effects. That is, the registration control amount establish processing (step) is interrupted in the presence of a cause of interruption, such as a cover of the apparatus getting open and a power source of the apparatus getting turned off. As the cause of interruption is eliminated and the interruption is resolved later, images are formed as usual immediately after this. Hence, as compared with where the registration control amount establish processing is to be executed once again after the interruption is resolved, the apparatus performs better. In addition, although the registration control amount establish processing (step) is not executed again after the interruption is resolved, since the registration control amount has been already calculated from the data acquired prior to the interruption, the transfer start positions of toner images are corrected for the respective toner colors in accordance with the registration control amount. Hence, a high-quality color image is obtained while suppressing registration deviations.
Further, it is possible to change a registration control amount in accordance with a necessity. Hence, it is possible to suppress a registration deviation while meeting a request from a user, by means of a proper change in registration control amount in response to the user""s request.
Further, the present invention makes it possible to selectively execute a registration control mode and a registration priority mode so that the abutting means contacts and moves away from the transfer medium under control in the selected mode. The registration priority mode as herein referred to is an operation mode which requires to rotate the transfer medium idle at least once or more times between first processing which is the image create/transfer processing in the last toner color and second processing which is the image create/transfer processing to form the next toner image and to cause the abutting means to temporarily contact the transfer medium during the idle rotation. Hence, when the registration priority mode is selected, registration deviations are prevented without fail in a manner which will be described in the section xe2x80x9cR. Eighteenth Preferred Embodimentxe2x80x9d later. Meanwhile, when the registration control mode is selected, since the abutting means is allowed to contact and move away from the transfer medium during repeated image create/transfer processing, a processing efficiency is more excellent and a throughput is higher than in the above-mentioned registration priority mode. Conversely, as described above, while registration deviations are created as the image create/transfer processing is executed with the transfer medium staying instable, the registration deviations are corrected in a manner similar to that in the invention described above and a high-quality image is obtained.